The search for satisfactory units of measurement in economics is never likely to attain complete success, any more than in other sciences. Statistical measurement of production requires both the identification of prime units or elements and the definition of categories into which the elements can be collected and which will support general statements about them. But the units that we use seldom have a clean-cut, unambiguous definition, and the categories into which they are aggregated continually shift, enlarge, change, divide, and dissolve.

The Standard Industrial Classification (SIC) represents one solution to this problem within its field of application. Those who constructed it have attempted to provide a suitable mixture of aggregates and particulars for the many uses to which the classification scheme will be put. The SIC has a far-reaching and pervasive influence on the forms and conditions in which economic information on production, sales, employment and investment becomes available. Its field of application is vast. It is a remarkable effort to impose an overall unity on diverse and heterogenous forms of data, in order to permit the numerous agencies and organizations collecting and using industry statistics to aggregate and compare them in terms of common categories.